Optimal Design of Natural and Hybrid Laminar Flow Control on Wings

by Pralits, Jan Oscar

Abstract (Summary)

Methods for optimal design of different means of control aredeveloped in this thesis. The main purpose is to maintain thelaminar flow on wings at a chord Reynolds number beyond what isusually transitional or turbulent. Linear stability analysis isused to compute the exponential amplification of infinitesimaldisturbances, which can be used to predict the location oflaminar-turbulent transition. The controls are computed usinggradient-based optimization techniques where the aim is tominimize an objective function based upon, or related to, thedisturbance growth. The gradients of the objective functionswith respect to the controls are evaluated from the solutionsof adjoint equations.Sensitivity analysis using the gradients of the disturbancekinetic energy with respect to different periodic forcing showwhere and by what means control is most efficiently made. Theresults are presented for flat plate boundary layer flows withdifferent free stream Mach numbers.A method to compute optimal steady suction distributions tominimize the disturbance kinetic energy is presented for bothincompressible and compressible boundary layer flows. It isshown how to formulate an objective function in order tominimize simultaneously different types of disturbances whichmight exist in two, and three-dimensional boundary layer flows.The problem formulation also includes control by means ofrealistic pressure chambers, and results are presented wherethe method is applied on a swept wing designed for commercialaircraft.Optimal temperature distributions for disturbance controlare presented for flat plate boundary layer flows. It is shownthat the efficiency of the control depends both on the freestream Mach number, and whether the wall downstream of thecontrol domain is insulated, or heat transfer occurs.Shape optimization is presented with the aim of reducing theaerodynamic drag, while maintaining operational properties.Results of optimized airfoils are presented for cases whereboth the disturbance kinetic energy, and wave drag are reducedsimultaneously while lift, and pitch-moment coefficients aswell as the volume are kept at desired values.Keywords:fluid mechanics, laminar-turbulent transition,boundary layer, laminar flow control, natural laminar flow,adjoint equations, optimal control, objective function, PSE,APSE, ABLE, HLFC, eN-method, Euler equations